The present invention is related to wafer processing and in particular to a method of monitoring a deposition or an etch process. More specifically, the present invention relates to a method of measuring the thickness of a film layer being deposited by a plasma enhanced chemical vapor deposition process (PECVD) or etched by a plasma etch process. The present invention may be used to measure the deposition or etch rate and film thickness of a variety of optically transparent films.
To provide repeatable device performance, the thickness of film layers must be maintained within specified tolerances. One methodology for determining the thickness of a film layer being deposited is to time the deposition and factor the time period with a theoretical rate to approximate the film thickness. To establish the actual deposition rate in a chamber, a semiconductor manufacturer processes and subsequently measures a film deposited on a few test wafers. The film thickness on actual production wafers is then assumed to approximately equal the measured deposition rate times the processing time. This process is time consuming and potentially results in wafers that do not meet the production specifications, as process drift may result in variable deposition rates.
Additionally, present process control is performed between two process steps. For example, a deposited film's properties such as thickness, stress, refractive index (RI), uniformity, and etch rate, which are important parameters characterizing the quality of a dielectric layer, are measured on a number of test wafers after the deposition or the etching of the layer on the test wafers. These test wafers are processed within a group of production wafers in order to assess whether the process step was performed within the desired specification for the entire group of wafers from which the test wafers were selected. This method can result in a substantial waste, since deviations from desired process parameters, and thus the production of film layers having non-optimal or undesirable properties, are only detected after a whole group of wafers has been processed. An additional limitation of the current test wafer thickness monitoring is that it is only capable of disclosing whether a particular film layer possesses the correct thickness, not why a film layer might deviate from a desired thickness. Accordingly, periodic process quality assessment methods are not equipped to provide information regarding potential counter measures to take to bring a process back in compliance with specifications. Therefore, a need to develop an in-situ characterization and process control exists.
Various in-situ systems for measuring deposition or etch rates have been developed by several research organizations and semiconductor manufacturers. These include single wavelength reflectometers, single wavelength ellipsometers, spectral ellipsometers, and optical emission interferometers. In these methods and systems, etch rate, deposition rate, rate uniformity and hence film thickness and variations thereof are calculated from the observed periodic modulations in the reflected light intensity. The origins of the modulations in the reflected light intensity and changes in its polarization arise from interference caused by the relative phase shift in the light reflected from the top and the bottom of the thin film. Systems employing light interference to measure deposition and etch rates typically include a light source and a detector and analyzer to detect and analyze the reflected light. The light detection can be either monochromatic (single wavelength) or spectral, and it can be carried out by many types of light sensors. Typically light from an optical source is conveyed to the detection equipment via an optical fiber. One class of these in-situ systems relies on optical emission interferometers (OEI) to monitor the wafer state during film deposition. OEI use light from the plasma as the source of incident radiation (i.e., the source of light upon the wafer), thereby reducing the cost and complexity of the wafer state monitor system. Other methods use an external light source along with or in lieu of the plasma emissions as the source of light upon the wafer.
However, most of these methods and systems require the source of incident radiation to be preferably perpendicular to the wafer surface. This also requires the detection equipment to be arranged in a manner to receive the reflected light, which is also nearly perpendicular to the wafer surface. This perpendicular or near perpendicular incidence requires the light source and detection equipment to be located opposed to the substrate's upper surface, which is typically reserved for other chamber components. Such a requirement may necessitate modifications to the design and geometry of some substrate processing chambers. Although some have been able to extend their system's angle of incidence (for detection) to as high as 52 degrees (measured with respect to wafer surface normal), such a setup still requires a large space between the inner top surface of the chamber and the wafer, to allow the reflected light to reach a detector.
Further, these geometric constraints that may require modifications to the chamber design, are further compounded by the needs of some existing systems to use expensive and complicated optical lens systems to gather and direct the light to the detection and analysis equipment.
Therefore, there is a need to develop a cost effective and easily implementable system for in-situ characterization and control of etch and deposition processes.